1. Field of the Invention
The present invention relates generally to demodulators for digital receivers, and more particularly, to a demodulator for digital radio receivers operating in severe noise/interference environment with multi-path propagation conditions.
2. Description of Related Art
Propagation conditions in mobile communications are often described as a multi-path fading channel model. In this model, the signal arriving at a receiver is a combination of rays, each of the rays is a replica of the original signal at the output of the transmitter but has different strength and epoch. The relative delay between the first ray and the last ray can be several symbol (or information bit) intervals. For example, it can be up to 5 symbol intervals in Global Systems for Mobile communications (GSM).
In order to compensate for a largely dispersive multipath fading channel, demodulators having a Viterbi equalizer (which utilizes Maximum Likelihood Sequence Estimation) are widely used (U.S. Pat. No. 5,091,918 granted to Wales, U.S. Pat. No. 5,285,480 granted to Chennakeshu et al. and U.S. Pat. No. 5,581,581 granted to Sato).
The Viterbi equalizer uses a propagation channel estimator to generate all possible signal sequences which result from being transmitted through the estimated propagation channel. Due to the channel memory length, the number of such signal sequences is limited. These generated signal sequences are then compared with a received signal sequence. The generated signal sequence which is closest in Euclidean distance to the received signal is selected. The data sequence associated with the selected signal sequence is the recovered data sequence. Propagation channel estimation is performed by detecting a known data sequence called midamble which is embedded in the middle of a signal burst. Good estimation of the propagation channel is crucial for the performance of the Viterbi equalizer. When the propagation channel becomes very noisy and/or when there are strong interfering signals (both cases are common in the mobile communication environment), the performance of the Viterbi equalizer can be significantly compromised since good estimation of the propagation channel is not available. Furthermore, the Viterbi equalizer is complicated to implement.
Accordingly, there is a need for a demodulator which can perform as well as or better than a demodulator having a Viterbi equalizer, and is easier to implement.
The present invention is a method and an apparatus for demodulating a signal received over a propagation channel. The received signal includes a synchronizing signal and is formed by one or more rays including a dominant ray. The method comprises the following: (a) generating a multi-path intensity profile of the propagation channel from the received signal; (b) determining an arrival time of the dominant ray and a propagation channel response corresponding to the arrival time based on the multi-path intensity profile; (c) decimating the received signal synchronously with the arrival time of the dominant ray to provide signal samples to an equalizer; (d) configuring the equalizer based on the multi-path intensity profile to deactivate some of the coefficients of the equalizer; (e) initializing the active coefficients of the equalizer based on the propagation channel response; (f) updating the active coefficients of the equalizer; and (g) equalizing the received signal using the equalizer to produce a filtered signal.